The functions of respiratory system progressively decline with advancing age even in healthy individuals. This decline is marked beyond 60 years of age. It is very difficult to ascribe these changes to age alone as lungs are exposed to various irritants during life such as tobacco smoke, respiratory infections, and air pollutants including dusts and fumes. In addition, sedentary life-styles and decreasing fitness with age significantly impair the respiratory functions. Despite these fallacies, age related changes in the respiratory system have been documented in the elderly, which can be measured objectively.

Though the routine activities of the elderly are not affected by declining functions of respiratory system, the reserve functions do decline. As a result of this, the elderly individuals become symptomatic with minimal stress.

Importance of knowing age related changes in respiratory system :

1. To know the difference between normal and abnormal respiratory functions in the elderly.

2. To know the presentation, course, treatment and ultimate prognosis of a particular disease which can be different from a young individual.

3. To know the predisposition to specific diseases in old age.

Basically, there occur structural changes in the respiratory system with advancing age, which will lead on to functional changes.

Structural Changes

1. Conducting Airways : They consist of the air passages from mouth to respiratory bronchioles and their volume comprises anatomic dead space. The larger airways increase in size as age advances, as a result of increase in cartilage size. This will result in a signifiant increase in anatomical dead space. There is calcification of cartilages and hypertrophy of bronchial mucosal glands with little or no physiological significance.

2. Lung Parenchyma: After 30 years of age, the respiratory bronchioles and alveolar ducts undergo progressive enlargement, which is termed as ductectasia. As a result of this, the area of alveolar ducts increases and alveolar septae become shortened leading to a flattened appearance of the alveoli. The net result of these changes is a decrease in the surface area of the alveoli by 15% by 60 years of age.

3. Chest Wall : The chest wall becomes more rigid as the articulations of the ribs with the sternum and spinal column become calcified with advancing age. This rigidity along with kyphosis due to osteoporosis decreases the compliance of the chest wall. The decreasing compliance of the chest wall demands greater work from respiratory muscles.

4. Muscles of Respiration : The maximal inspiratory pressure (Pimax) and maximal expiratory pressure (Pemax) decrease as the muscle strength progressively decreases with advancing age. The various factors that contribute to the decrease in muscle strength are:

a) Atrophy of motor units

b) Decreased efficiency of energy utilization

c) Obesity, and

d) Sedentary life style

5. Pulmonary Vasculature: In the elderly individuals with normal health, there is increased pulmonary vascular resistance and increased pulmonary artery wedge pressure during exercise. However, these changes are physiologically insignificant.

Physiological Changes

1. Mechanical Properties

Though there is an absolute increase in the elastin content of the lungs as age advances, the elasticity of the lung parenchyma decreases because of the structural alterations in elastin fibres. As a result of this decreased elasticity and decreased surface tension of alveoli, elastic recoil of the lung decreases and the transpulmonary pressure (TPP) falls at a given lung volume

2. Breathing Control

There is decreased ventilatory response to hypoxia and hypercapina as age advances. It is not clear whether these changes are due to alterations in the sensitivity of respiratory centre or chemoreceptors. In addition, there is enhanced sensation of dyspnea with reduced compensatory response.

3. Pulmonary Function Tests

Lung function and exercise capacity decline with age as a result of morphological and physiolgoical changes. But documentation and standardization of these age related changes are very difficult because of confounding factors such as increased prevalence of disease, chronic illness, use of medication, sedentary life style and difference in the physiological and chronological age. Despite these difficulties, age related changes in pulmonary function have been documented by cross-sectional and longitudinal studies.

Lung Volumes

1. Total Lung Capacity (TLC) : As the compliance and Pimax decrease with age, TLC also decreases. Cross sectional studies have shown an average decline of 25 ml/yr which in statistically insignificant.

2. Vital Capacity (VC): Both longitudinal and cross sectional studies have shown progressive decline in both forced and slow VC with advancing age. The average decline is 21-33 ml/yr in males and 18-29 ml/yr in females.

3. Residual Volume (RV) : As age advances, the expiration of lung volume is incomplete as a result of decreases in elastic recoil, chest wall compliance, Pemax and increase in closing volume due to small airway closure at a higher volume. This leads on to significant increases in RV and RV/TLC. But the increase in FRC in clinically insignificant.

2. Peak Expiratory Flow (PFF) : PEF shows a nonlinear decline with advancing age. It progressively increases till 30-35 years of age, then attains a plateau phase and start declining after 50 years of age (4 l/min/yr in males and 2.5 l/min/yr in females.

3. FEV1 : FEV1 also follows the pattern of PEF as the age advances. But the decline in FEV1 after the age of 50 years is more rapid in individuals with tall stature, larger baseline expiratory flow and airway reactivity.

Airway Resistance

There is no overall change in the resistance of airways. The increase in resistance in peripheral airways due to decreasing airway size is compensated by the decreased resistance in larger airways.

Gas Exchange

1. DLCO : The linear decline in DLCO is due to decrease in diffusion capacity and diminished pulmonary blood flow in the elderly.

2. PaO2 - As age advances, there is closure of airway at a lower tidal volume and increase in the physiological dead space leading on to V/Q mismatch and hence arterial hypoxaemia.

3. PA O2 - Normal

4. P (A-a)O2 - Increased

5. PCO2, pH - Normal

Interpretation of Lung Function Tests in the Elderly

A clinician must be cautious while interpreting a pulmonary function test of an elderly individual because:

1. There are no accepted predicted normal values for these individuals.

2. Co-existent illnesses in elderly, and

3. There is only a marginal difference between normal and abnormal values.

The knowledge about changes in the aging respiratory system is essential while interpreting the clinical features, imaging and laboratory data of fragile elderly individuals, as correct clinical decision making significantly influences the mortality and morbidity.